Wafer edge polishing method and apparatus

ABSTRACT

A semi-conductor wafer ( 12 ) has its inclined edge flanks polished by a grooved wheel ( 16 ) of synthetic plastics material, while a jet ( 20 ) of polishing slurry, which may comprise colloidal silica, is fed downwards into the zone of contact between the wheel and the wafer. The wheel ( 16 ) is preferably rocked or oscillated laterally such that a constant polishing force is alternatively applied to each flank of the wafer ( 12 ). The wheel may be mounted in a buffer store to which the wafers are transported from a grinding station which grinds the edge flanks.

FIELD OF INVENTION

This invention concerns the polishing of the edges of semi-conductorwafers such as silicon wafers and apparatus therefor.

BACKGROUND OF THE INVENTION

When silicon has been crystallised into large boules, which aregenerally cylindrical over most of their length, the crystallinematerial is sliced into thin circular discs known as silicon wafers, thediameter of each of which can be anything within the range 25-300 mm.After mounting in a vacuum chuck in an edge grinding machine, theperipheral edges of the discs are accurately ground so as to produce anaccurate circular shape centered on the centre of the disc and aprecisely formed edge profile, and are often notched at one positionaround the circular periphery.

Whereas over the majority of the area of disc the thickness is uniformand the two faces are parallel, the periphery of such discs are groundto a triangular section in which the apex of the triangle defines theoutermost diameter of the wafer and is normally located midway betweenthe two parallel faces of the disc.

In order to optimise the manufacture of semi-conductor devices usingsuch a disc, it is important that the surface which is to be exposed tophoto-lithography is damage free and in view of the microscopic size ofthe microcircuits which are formed on the wafers, damage near to thecircumferential line between which the flat circular surface merges withthe inclined surface leading to the apex, can significantly reduce thenumber of devices which can be made from the wafer.

In machines which do not produce a good quality ground finish to theinclined surfaces of the triangular section leading to the apex, it isnecessary to etch and then polish the edges of the wafer before it canbe used in the construction of semi-conductor devices. Where a goodquality ground finish is produced on the inclined surfaces leading tothe apex, it is only necessary to etch and polish a small quantity ofmaterial in order to obtain an undamaged edge.

It is an object of the present invention to provide an improved methodand apparatus for polishing a finely ground wafer edge.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided amethod of edge grinding and polishing circular workpieces which aretransported in turn to a grinding station in which the edge of theworkpiece is ground, and after grinding, the workpiece is transferred toa buffer store whilst the next workpiece is conveyed to and located inthe grinding station for grinding, which further comprises the steps ofrotating the workpiece in the buffer store and engaging the edge thereofwith a polishing wheel, to polish its periphery whilst the nextworkpiece is being ground, and after polishing, the polished workpieceis removed and the said next wafer is transferred from the grindingstation to the buffer store for polishing to allow a further workpieceto be located in the grinding station.

The workpieces awaiting grinding may be contained in a first store andthe ground and polished workpieces may be contained in a second store.The first and second stores may be different sections of a singlestorage device.

The invention is particularly applicable to circular semi-conductorwafers.

The buffer store conveniently comprises a wash station and in accordancewith a preferred aspect of the invention, a method of edge grinding andpolishing wafers as aforesaid includes the step of washing the groundwafer before the polishing step.

The invention also lies in a method in which the wafer is washed afterit has been polished.

The invention also lies in a method in which the wafer is washed bothbefore and after the polishing step.

In accordance with a preferred feature of the invention, the methodincludes the step of forming and reforming a groove in the polishingwheel after one or more polishing steps have been performed.

The forming and reforming of the groove in the wheel may be effected bymeans of a groove-forming wheel having a triangular cross-sectionperipheral rim, defining in cross-section the two slantingfrusto-conical edges which converge in an apex, and after polishing oneor more wafers, relative movement is effected between the groove-formingwheel and the polishing wheel whilst both are rotating about theirrespective axes, thereby to form or reform the cross-sectional shape ofthe groove in the polishing wheel ready for polishing the edges offurther wafers.

According to another aspect of the invention, apparatus for grinding theedges of circular wafers of semi-conductor material comprises a storewithin which disc-like wafers of semi-conductor material can be stored,a grinding station, means for conveying the wafers one at a time to thegrinding station for effecting edge grinding thereof and after grindingconveying each wafer in turn to a buffer store and locating the wafer ona storage chuck therein, said conveying means selecting the next waferfrom the store and conveying it to the grinding station to effect edgegrinding thereof, a polishing wheel in the buffer store, means forrotating it and the wafer therein, means for effecting relative movementbetween the wafer and the polishing wheel to polish the edge of thewafer whilst the next wafer is being ground, and said conveying meansserving to remove the polished wafer from the buffer store and selectinga further wafer from the store and conveying it to the grinding stationfor edge grinding thereof, before moving to the buffer store to recoverthe polished wafer from therein.

Preferably in methods and apparatus embodying the invention, a slurry isconveyed to the point of contact between the wafer and the polishingwheel so as to assist in the polishing action.

Typically the polishing wheel is compliant and is formed from a materialsuch as urethane or serium oxide.

Typically the slurry comprises an alkali solution, preferably of pH11.

The method of the invention may preferably include the further step ofeffecting a rocking movement between the polishing wheel and the waferedge by rocking either the wheel or the wafer about an axis, coincidentwith or parallel to a tangent which intersects the point of contactbetween the wafer and the polishing wheel.

In a preferred arrangement the wafer and the grooved polishing wheel aremounted for rotation about horizontally spaced apart parallel axes, thewafer is supported on a vacuum chuck and is rotated by a drive motor,the polishing wheel is driven in rotation by a second drive means,linear drive means is provided to advance and retract the polishingwheel relative to the wafer, and oscillatory drive means acts on theassembly of polishing wheel and associated drive so as to rock the wheelabout a vertical axis so that the two side cheeks of the groove in thepolishing wheel alternately engage the inclined edges of the edge regionof the wafer.

In an alternative arrangement the wheel and its rotational drive remainsstationary except for the rotation of the wheel about its axis, andlinear translation means is provided for advancing the wafer andassociated drive and support means towards and away from the groovedpolishing wheel to engage the edge of the wafer in the groove, andoscillatory drive means serves to rock the wafer support and thereforethe wafer thereon about a vertical axis so as to produce the alternatingengagement between the external periphery of the wafer and the sides ofthe groove as the wafer and polishing wheel rotate.

References to oscillatory drive means include rocking drives and driveswhich rotate first in one direction and then the other about a singleaxis.

According to a further aspect of the invention, it is to be understoodthat the technique of rocking the wafer or the wheel whilst rotating soas to alternately engage the two inclined faces of the periphery of thewafer with the corresponding faces of the groove in the wheel, is notlinked to an arrangement in which the wafer is mounted in a buffer storeon, or associated with, an edge grinding machine. This further aspect ofthe invention can be incorporated into any wafer edge polishing machinein which either the wafer or the wheel can be adapted to be rockedalternately in one direction and then the other about an appropriateaxis.

The invention therefore also lies in apparatus for wafer edge polishingin which there is provided means for rotating a wafer, a polishing wheelhaving a groove therein for accommodating the peripheral rim of thewafer, means for rotating the wheel, drive means for effecting relativemovement between the wheel and the wafer to effect engagement of thewafer in the groove, and further drive means to effect relative rockingmovement between the wafer and the wheel such that opposite faces of theperipheral rim of the wafer are engaged alternately by opposite regionsof the surface of the groove in the wheel during the polishing process.

In any of the apparatus as aforesaid, a jet orifice is located near tothe region of engagement between the wafer and the wheel, for directinga fluid slurry towards the region of engagement at least during thepolishing process.

According to another feature of the invention, in any of the apparatusas aforesaid, a groove-forming wheel (a grooving tool) is mounted forengagement with the groove in the polishing wheel either by movement ofthe grooving tool towards the wheel or the wheel towards the tool, andfor mutually rotating both wheel and tool so as to form or reform thegroove in the surface of the wheel.

Where the grooving tool is a disc, the latter may be mounted to the rearof the wafer support for rotation therewith, and the forming step isperformed by moving the grooved wheel into engagement with the groovingtool and rotating both so as to form or reform the groove in the wheel.

The invention also lies in any of the methods as aforesaid furthercomprising the step of projecting a fluid slurry towards the point ofengagement between the grooved polishing wheel and the periphery of thewafer at least during polishing engagement therebetween.

The invention also lies in any of the methods as aforesaid in which agrooving tool (groove-forming wheel) is located in proximity to thegrooved polishing wheel for engagement therewith to form and reform thegroove in the wheel as required.

According to a further aspect of the invention, the polishing wheel hasa significant axial depth, and two or more parallel grooves are formedtherearound, so that as one groove becomes worn and needs reforming, thewheel can be indexed so as to use another of the grooves for polishingthe edge of the wafer and the process of moving from one groove toanother can be continued until all of the grooves have been utilised andneed reforming before the process is arrested and a reforming process isperformed on the polishing wheel grooves.

According to another aspect of the present invention, in a method ofpolishing the edge of a disc-like workpiece the latter is secured inposition on a vacuum chuck for rotation about its centre, a polishingwheel of synthetic plastics material is brought into contact with therotating edge of the workpiece having previously been formed around itspolishing surface with a groove the cross-section of which is thecomplement of the edge profile of the disc, a polishing slurry issupplied to the zone of contact between the polishing wheel and the edgeof the workpiece, and the polishing wheel is rotated so that relativemovement exists between the polishing wheel and the disc edge.

Typically the polishing wheel is rotated contra to the direction ofrotation of the workpiece in the zone of contact.

Typically the synthetics plastics material is polyurethane.

Typically the polishing slurry is colloidal silica.

Whereas with edge grinding, the grooved grinding wheel is advancedduring the grinding process with a controlled feed rate, it has beenfound more preferable to advance a synthetic plastics polishing wheel insuch a manner as to engage the edge of the workpiece in a so-calledplunge mode and under constant force.

A polished surface has been obtained in a matter of a few minutes,typically of the order of 2 to 4 minutes.

The method may be performed as a modified edge grinding machine whichthereby permits the disc to be edge ground and then polished while stillmounted on the same chuck, by withdrawing the grinding wheel andadvancing the polishing wheel and thereafter performing the aforesaidmethod. A washing step may be included between the grinding step an thepolishing step.

Preferably the method is under the control of a computer programmed toinstigate the different steps of the process.

According to another aspect of the present invention apparatus forperforming the aforementioned method comprises a vacuum chuck forreceiving and supporting a circular plate-like workpiece, drive meansfor rotating the chuck and therefore the workpiece about its centre,means for mounting a grooved polishing wheel and means for rotating sameabout its central axis so that its direction of rotation is contra thatof the rotation of the workpiece in the region in which they will makecontact, drive means for advancing and retracting the polishing wheelmounting means to enable the polishing wheel to make contact with theedge of the workpiece for polishing purposes, and means for adjustingthe polishing wheel relative to the mounting means or the mounting meansrelative to the remainder of the apparatus, or both, to ensure that thegroove in the surface of the polishing wheel accurately aligns with theedge of the workpiece, the apparatus further comprising means forsupplying a polishing slurry to the zone of contact between theworkpiece and the polishing wheel.

Preferably the apparatus further includes a forming wheel mounted forrotation about its central axis typically in a contra sense to therotation of the polishing wheel where the two will come into contact,and means is provided for effecting relative movement between thepolishing wheel and the forming wheel to engage the edge of the formingwheel with a surface of the polishing wheel to form a groove therein,the external periphery of the forming wheel corresponding to theexternal edge profile of the workpiece so that the groove in thepolishing wheel corresponds to the edge profile of the workpiece.

In apparatus as aforesaid, the polishing wheel may have significantaxial extent in the form of a cylinder, whereby as one grooved regionthereof wears away as a result of the polishing activity, one or morefurther grooves can be formed at axially spaced positions therealong byappropriate indexing of either the forming wheel or the polishing wheelor both, and the apparatus is further programmed to introduce relativeindexing between the forming wheel and polishing wheel and between thelatter and the workpiece so that the appropriate groove in the surfaceof the polishing wheel is employed to polish the edge of the workpiece.

The means for supplying polishing slurry to the polishing zonepreferably includes at least one nozzle for directing the slurry towardsthe edge of the workpiece.

Typically the nozzle or nozzles direct the slurry towards the smallregion of contact which exists between the polishing wheel and theworkpiece.

Where the polishing wheel rotates significantly faster than theworkpiece, it may assist if the slurry is directed towards the region ofcontact so as to be moving in the same direction as the periphery of thepolishing wheel at the point of contact. Since the groove within thepolishing wheel forms a small reservoir with the edge of the workpiecewhen the two are engaged, the rotation of the workpiece and polishingwheel may be selected so that the perimeter of the polishing wheel ismoving in a downward sense at the point of contact so that the slurrycan be projected down into the junction between the polishing wheel andthe edge of the workpiece from above, so that a puddle of slurry ismaintained in the small reservoir as aforesaid times during polishing.

Preferably the polishing action occurs within a generally closedenvironment so that slurry which is spattered away from the rotatingparts is collected on the walls of the enclosure from which it can draininto a collection sump.

Preferably means is provided for draining slurry from the sump into areservoir from which it can be drained or pumped for disposal orrecovered for re-use.

Filtering means is preferably provided to remove particles from theslurry which are greater than a given size if the recovered slurry is tobe reused.

Preferably means is provided for sensing the flow of slurry through thenozzle which includes an interlock for retracting the polishing wheel todisengage it from the workpiece in the event that the slurry flow ceasesor drops below a predetermined flow rate.

In apparatus as aforesaid, where the polishing wheel is polyurethane, apreferred slurry is colloidal silica.

A preferred material for the polishing wheel is polyurethane.

The apparatus as aforesaid may comprise a wafer grinding machine inwhich one or more grinding wheels are advanced under computer controlinto engagement with the workpiece to effect an initial (or finish)grinding of the edge, and on which machine the polishing wheel is alsomounted, and the polishing wheel again under computer control, isadvanced into contact with the edge of the workpiece after the grindingprocess has been completed, to perform a polishing stage.

Where the polishing wheel is located on the grinding machine, thegrinding wheel and polishing wheel may be mounted on the same wheelheadassembly for rotation by a common drive means such as an electric,hydraulic or pneumatic motor.

Where the same drive is used, the speed of rotation may be varied asbetween grinding and polishing as appropriate.

Furthermore, where the same advance and retract mechanism is used foradvancing and retracting the grinding wheel and polishing wheel, thecomputer is further programmed to alter the feed characteristics fromwhen the grinding wheel is advanced, to a constant force plunge mode forwhen the polishing wheel is to be used.

Since the grinding process will produce a considerable quantity of swarfand spatter during the grinding process, the grinding process ispreferably carried out within a similar enclosure as is the preferredarrangement for the polishing step, and where the same enclosure isused, preferably a washing step is included between the grinding andpolishing steps so as to wash the interior of the enclosure and moreparticularly at least the surface of the polishing wheel, to remove anytraces of grinding swarf, before the polishing step is performed.

In addition or instead, a protective sleeve or housing may be providedfor covering the polishing wheel except when the latter is to be madeavailable for polishing, so that there is little or no chance of anygrinding swarf finding its way onto the surface of the polishing wheel.

Where a sleeve or housing shrouds the polishing wheel during thegrinding step, the shrouding enclosure is preferably openable andmovable under control of the computer after the grinding process hasceased and any washing step has been performed to expose the polishingwheel and allow the latter to move into its polishing position.

The invention will now be described, by way of example, with referenceto the accompanying drawings in which:

FIG. 1 is a diagrammatic elevation of a polishing wash station for usein combination with a wafer edge grinding apparatus;

FIG. 2 is a diagrammatic top plan view of apparatus such as shown inFIG. 1; and

FIG. 3 is a diagrammatic view in elevation, showing a modified mechanismfor causing the polishing wheel to execute an oscillating motion.

The invention is applicable to the polishing of semi-conductor waferswhich have been edge ground by an edge grinder such as described in UKPatent Specification No. 2317585.

The apparatus illustrated diagrammatically in FIG. 1 comprises a housing10 within which is located a wafer 12 mounted on a vacuum chuck adaptedfor rotation about an axis 14, and a urethane polishing wheel 16 mountedfor rotation about parallel axis 18.

A shallow V-shaped groove 33 is formed around the periphery of thepolishing wheel 16. The wheel and the wafer are rotated as relativemovement brings the two into engagement, so that the triangular sectionperiphery of the wafer enters the groove 33 around the wheel. Both thewheel and the wafer are rotated clockwise, as shown by the arrows, sothat there is contrary motion at the point of engagement.

A jet 20 projects a fluid polishing slurry downwards in the direction ofmotion of the peripheral region of the wheel 16 which will engage thewafer 12. The slurry may typically be an alkali solution of pH11 withcolloidal silica having a mean particle size of 30-50 nm: for example aslurry known as Klubesol 50R50 manufactured by the company Clariant anddiluted 10:1 with water.

Remote from the wafer is located a groove-forming wheel 22 rotatableabout an axis 24. Relative movement between forming wheel 22 andpolishing wheel 16 brings the edge of the wheel 22 into contact with thewheel 16 and into the groove 33 around the wheel. The peripheral shapeof the forming wheel 22 is similar to the peripheral cross-sectionalshape of the wafers such as 12 which are to be polished by the groove inthe polishing wheel 16, and by forming the wheel 22 from a hard materialand the polishing wheel 16 from a compliant material such as urethane,the groove 33 around the wheel will be formed or re-formed due to theengagement between the forming wheel and the groove in the polishingwheel.

As an alternative to the forming wheel 22, there may be provided anon-rotatable profiled turning tool 23, (shown in chain-dotted outline),which in operation is slidable in the direction of arrow 25 intoengagement with the wheel 16 for forming or re-forming the groove 33.

FIG. 2 shows the apparatus of FIG. 1 in more detail and from above.

The wafer 12 is shown mounted on a vacuum chuck 26 carried by a spindle28 driven in rotation by a motor 30. Optionally mounted on the shaft 28(in place of wheel 22) is a groove-forming wheel 32 having a profiledperiphery adapted to form the V-shaped groove 33 in the cylindrical faceof the polishing wheel 16.

Polishing wheel 16 is carried by a shaft 34 driven in rotation by amotor 36.

The motor 36 is mounted on a slideway and is laterally slidabletherealong under the action of an air cylinder 38 so as to effectengagement of the wafer 12 with the groove 33 or by appropriate axialmovement by means of another drive 40, in the direction of the arrow 42,to effect engagement between the groove 33 and the groove-forming wheel32.

The motor 36 is mounted for pivotal motion about a generally verticalaxis relative to its slideways, and a lever 44 is shown extending fromone side of the motor housing acted on by two piston and cylinder drivearrangements 46 and 48 which are selectively supplied with fluid such asair via a valve 50 from an air line 52.

By alternately supplying fluid to one and then the other of the drives46, 48, so the pistons are driven in first one direction and then theother. This oscillates the lever 44, and therefore the motor attachedthereto, so as to produce a rocking motion about the axis of pivoting.

Ideally the polishing wheel and drive assembly is carried by upper andlower pivoting arms which transfer the pivoting motion to an axis whichextends substantially through the centre of the polishing wheel in agenerally vertical sense. To this end the motor housing may be mountedbetween upper and lower arms which protrude therefrom in a forward senseabove and below the polishing wheel 16 and vertically aligned pivotbearings are provided above and below the wheel to which the arms areattached and about which the arm assembly can swivel to effect therocking motion of the wheel 16 relative to the wafer 12, about thevertical axis.

The rocking motion polishes the flanks of the triangular cross-sectionaledge region of the wafer 12.

FIG. 3 shows in elevation a modified arrangement for causing thepolishing wheel 16 to execute an oscillating motion relative to thewafer (not shown). The wheel is provided with two grooves 33 A and 33 Bso that when one becomes worn the other can be utilised.

Here the wheel 16 is mounted on shaft 34 and supported by trunnions 60,62 on a slideway 64 which is mounted horizontally parallel to therotational axis of the wafer. The shaft 34 is again driven in rotationby a motor (not shown).

Projecting downwards from the slideway 64 is a spigot 66 whose oppositesides are engaged alternatively by push rods 68, 70 of respective aircylinders 72, 74, which in turn are mounted via supports 73 on aslidable plate 76. Horizontal movement of the plate, and hence of thecylinders, is controlled by a motor driven cam 78, such as an eccentric,engagable with rotatable cam followers 80, 82 whose spindles 84, 86 areconnected to the plate 76.

If operation, rotation of the cam 78 causes spigot 66 to be engagedalternately by the two rods 68, 70. The air pressure P in the cylindersis regulated such that the lateral force exerted on the wafer flanks isconstant, regardless of the actual displacement of the cylinder rods.

What is claimed is:
 1. A method of edge grinding and polishingsemi-conductor wafers which are transported in turn to a grindingstation in which the edge of the wafer is ground, and the wafer is thentransferred to a buffer store whilst the next wafer is conveyed to andlocated in the grinding station for grinding, which further comprisesthe steps of rotating the wafer in the buffer store and engaging theedge thereof with a polishing wheel, to polish its periphery whilst thenext wafer is being ground, and after polishing, the polished wafer isremoved and the next wafer is transferred from the grinding station tothe buffer store for polishing to allow a further wafer to be located inthe grinding station, wherein the polishing step is effected by arocking movement between the polishing wheel and the wafer edge byrocking either the wheel or the wafer about an axis, coincident with orparallel to a tangent which intersects the point of contact between thewafer and the polishing wheel.
 2. A method according to claim 1 in whichthe wafers awaiting grinding are contained in a first store and theground and polished wafers are contained in a second store.
 3. A methodaccording to claim 2 in which the first and second stores are differentsections of a single storage device.
 4. A method according to claim 1 inwhich the buffer store comprises a wash station and further comprisingthe step of washing the ground wafer before the polishing step.
 5. Amethod according to claim 1 in which the wafer is washed after it hasbeen polished.
 6. A method according to claim 1, further including thestep of forming and reforming a groove in the polishing wheel after oneor more polishing steps have been performed.
 7. A method according toclaim 6 in which the forming and reforming of the groove in thepolishing wheel is effected by means of a groove-forming wheel having atriangular cross-section peripheral rim, defining in cross-section twoslanting frusto-conical edges which converge in an apex, and afterpolishing one or more wafers, relative movement is effected between thegroove-forming wheel and the polishing wheel whilst both are rotatingabout their respective axes, thereby to form or reform thecross-sectional shape of the groove in the polishing wheel ready forpolishing the edges of further wafers.
 8. A method according to claim 1further comprising the step of projecting a fluid slurry towards thepoint of engagement between the grooved polishing wheel and theperiphery of the wafer at least during polishing engagementtherebetween.
 9. A method according to claim 1 in which a grooving tool(groove-forming wheel) is located in proximity to the grooved polishingwheel for engagement therewith to form and reforn the groove in thewheel as required.
 10. A method according to claim 1 in which thepolishing wheel is made of synthetic plastics material and is broughtinto contact with the rotating edge of the wafer, said wheel havingpreviously been formed around its polishing surface with a groove thecross-section of which is the complement of the edge profile of thewafer, a polishing slurry is supplied to the zone of contact between thepolishing wheel and the edge of the wafer, and the polishing wheel isrotated so that relative movement exists between the polishing wheel andthe said edge.